1. Field of the Invention
The present invention relates to a variable focal length lens system and an imaging apparatus. More particularly, the present invention relates to a variable focal length lens system and an imaging apparatus that are used in, for example, a video camera or a digital still camera and have an angle of view of more than 70 degrees and a zoom ratio of more than 10.
2. Description of the Related Art
A method has been proposed which converts the quantity of light of an object image which is formed on an imaging device surface of an imaging device including a photoelectric conversion element, such as a CCD (charge coupled device) or a CMOS (complementary metal-oxide semiconductor), as an imaging unit of a camera into an electric output using each photoelectric conversion element and records the electric output.
With the progress of microfabrication techniques, the process speed of a central processing unit (CPU) or the degree of integration of a recording medium has increased, which makes it possible to process a large amount of image data at a high speed. In addition, with an increase in the degree of integration of light-receiving elements, it is possible to record data at a high spatial frequency. With a reduction in the size of the light-receiving element, it is possible to reduce the overall size of a camera.
However, when the degree of integration increases and size is reduced, the area of a light-receiving surface of each photoelectric conversion element is reduced and the electric output is lowered. As a result, the influence of noise on the photoelectric conversion element increases. In order to reduce the influence of noise, a structure has been proposed which increases the aperture ratio of an optical system to increase the amount of light incident on the light-receiving element. In addition, a structure has been proposed in which a micro lens element, which is called a micro lens array, is provided immediately before each element.
The micro lens array restricts the position of an exit pupil of the lens system, instead of guiding rays incident between adjacent elements onto the elements. When the position of the exit pupil of the lens system is close to the light-receiving element, an angle between a principal ray incident on the light-receiving element and the optical axis is increased. Therefore, an angle between an off-axis ray traveling to the periphery of a screen and the optical axis is increased. As a result, a necessary amount of light is not incident on the light-receiving element and the amount of light is insufficient.
In recent years, with the wide spread use of digital cameras, there have been various demands from the users.
In particular, there is demand for a small-sized camera with a high zoom ratio zoom lens (variable focal length lens system). A zoom lens having a zoom ratio of more than 10 has been provided.
In general, a zoom lens having a high zoom ratio includes four lens groups with positive, negative, positive, and positive refractive powers.
In the zoom lens including the four lens groups with positive, negative, positive, and positive refractive powers, four lens groups, that is, a first lens group with a positive refractive power, a second lens group with a negative refractive power, a third lens group with a positive refractive power, and a fourth lens group with a positive refractive power are arranged in this order from an object side to an image side. In the zoom lens including the four lens groups with positive, negative, positive, and positive refractive powers, when the position of the lens is changed from a wide-angle end with the shortest focal length to a telephoto end with the longest focal length, the first to third lens groups are moved such that the gap between the first lens group and the second lens group increases and the gap between the second lens group and the third lens group decreases. The fourth lens group is moved to compensate a variation in the position of an image surface.
For example, Japanese Unexamined Patent Application Publication No. 2008-146016 discloses such a zoom lens including four lens groups with positive, negative, positive, and positive refractive powers.
In recent years, wide-angle zoom lenses with an angle of view of more than 75 degrees have increased. As the wide-angle zoom lens, a zoom lens having a first lens group with a negative refractive power has been generally used.
For example, Japanese Unexamined Patent Application Publication No. 2007-94174 discloses a zoom lens in which two lens groups, that is, a first lens group with a negative refractive power and a second lens group with a positive refractive power, are arranged in this order from the object side to the image side.
Japanese Unexamined Patent Application Publication No. 2008-46208 discloses a zoom lens in which four lens groups, that is, a first lens group with a negative refractive power, a second lens group with a positive refractive power, a third lens group with a negative refractive power, and a fourth lens group with a positive refractive power, are arranged in this order from the object side to the image side.
In recent years, an aspheric lens has generally been used. A zoom lens including a first lens group with a positive refractive power has generally been used.
For example, Japanese Unexamined Patent Application Publication No. 2008-102165 discloses a zoom lens that includes a first lens group with a positive refractive power and uses aspheric lenses to achieve a wide angle of view and a high variable power.